ALSA: hda - Add pcm_mmap_prepare op.

Adding this op allows the X86 specific mmap operation to help in
hda_intel without needing a CONFIG_X86 in future non-PCI hda drivers.

Signed-off-by: Dylan Reid <dgreid@chromium.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>